GEOL 104 Dinosaurs: A Natural History

Fall Semester 2017
Dinosaurs Without Bones: Trace Fossil Analysis

Dinosaur footprints at the Mill Canyon Dinosaur Tracksite, Utah

Key Points:
•Cladistics (phylogenetic systematics) is a method for approximating the evolutionary relationships among taxa.
•Cladistics works by trying to reconstruct the pattern of common ancestry rather than finding direct ancestor-descendant relationships.
•Not all traits are equally useful for reconstructing phylogenetic relationships: only shared evolutionary transformations help us determine phylogenetic patterns.
•Phylogenetic information can be used as a basis for taxonomy; as a means of inferring missing and ancestral information; and for determining the time of divergence between lineages.

Ichnology: the study of trace fossils.
Most dinosaur ichnologists concentrate on footprints and trackway analysis.

An individual footprint represents:

Footprints give direct information about the soft tissues of the bottom of the foot, and about the natural position of the toes.

Trackways, however, give even more data. By measuring the stride length, and estimating hip height, the speed of the dinosaur at the time of that trackway can be calculated. These data tend to show dinosaurs walking around at speeds comparable to modern large-bodied mammals.

However, trackways do have some problems:

Footprints and trackways can, however, reveal the presence of dinosaurs not yet known by body fossils (such as Middle Jurassic North American dinosaurs).

There is a whole discipline of ichnotaxonomy: the naming of trace fossils. However, it must be remembered that these are sedimentological entities, not biological entities: the same animal can produce tracks given entirely different ichnotaxonomic names if it is walking slowly or running; on soft mud or hard mud; if adult or juvenile; etc.

One interesting note: almost no dinosaur trace fossil shows tail drag marks: this was some of the first evidence that dinosaurs held their tails up above the ground.

Trace fossils can help us understand something about group behavior. A recent study of Alaksan hadrosaurs showed many individuals moving in the same direction at the same time (based on the similar quality of footprint preservation.) Additionally, by counting up the trackways of different sizes, it helped to give an estimate of the relative fraction of the herd of different growth stages.

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Last modified: 5 July 2017

Dinosaur track in ripple-marked limestone at the Red Gulch tracksite, Middle Jurassic Sundance Formation, near Shell, WY